This invention relates to electromagnetic discharge apparatus and more particularly to electrodeless fluorescent light sources in which a low pressure mercury discharge contained in a phosphor coated envelope is excited by high frequency power in a termination fixture.
Conventional high brightness fluorescent lamps provide long life and efficient operation but require large, heavy, and expensive ballasting circuits for operation at line frequencies. Conversion to high frequency operation to reduce the size of ballasting circuits does not eliminate the problem because of the cost of discrete components and magnetic materials used in these circuits. An additional problem as one attempts to make small fluorescent lamps is that power losses connected with the electrodes become an increasingly large fraction of the applied power.
Hollister has shown a technique for excitation of phosphor coated low pressure electrodeless lamps in U.S. Pat. No. 4,010,400 issued Mar. 1, 1977.
According to Hollister's patent, radio frequency power, typically at a frequency of 4 MHz, is coupled to a discharge medium contained in a phosphor coated envelope by an induction coil connected to a radio frequency source. Upon excitation, the discharge medium emits radiation which in turn causes excitation of the phosphor to produce visible light. One drawback of this approach is that several relatively expensive discrete components, in particular the induction coil, are required. Furthermore, the induction coil acts as an antenna and produces a considerable amount of RFI (Radio Frequency Interference). Optically transparent shielding is relatively difficult in this frequency range. Moreover, allowable tolerances on operating frequencies as required by the FCC are low for the frequencies used by Hollister and may require the use of crystal controlled oscillators.
Electrodeless light sources which operate by coupling high frequency power, typically 915 MHz, to a high pressure arc discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor surrounding the inner conductor as described in U.S. Pat. No. 3,942,058 issued Mar. 2, 1976 to Haugsjaa et al. and U.S. Pat. No. 3,942,068 issued Mar. 2, 1976 to Haugsjaa et al. The electrodeless lamp is positioned at the end of the inner conductor and acts as a termination load for the fixture. The termination fixture has the function of matching the impedance of the electrodeless lamp during high pressure discharge to the output impedance of the high frequency power source. Thus, when the high pressure discharge reaches steady state, a high percentage of input high frequency power is absorbed by the discharge in the electrodeless lamp. One method of constructing a termination fixture which matches the electrodeless lamp to the power source is shown in U.S. Pat. No. 3,943,403 issued Mar. 9, 1976 to Haugsjaa et al. The inner conductor has a length equal to one quarter wavelength at the operating frequency. Located at the source end of the termination fixture, or one quarter wavelength from the electrodeless lamp, is a capacitor which compensates for the reactive component of the lamp impedance. The dimensions of the termination fixture are such that the complex electrodeless lamp impedance is matched to the source impedance. Another method of constructing a termination fixture which matches the electrodeless lamp to the power source is shown in U.S. Pat. No. 3,943,404 issued Mar. 9, 1976 to McNeill et al. A helical coil couples the inner conductor to the electrodeless lamp and compensates for the reactive component of the electrodeless lamp impedance.
Since the high pressure arc discharge provides usable light output directly, both the electrodeless lamp and the termination fixture must be capable of transmitting visible light. The light transmitting portion of the termination fixture typically includes a transparent dome covered with a conductive mesh. At the frequency of operation, typically 915 MHz, a fine mesh is effective as an RFI shield, and little of the light output is blocked. By contrast, at lower frequencies of operation, such as those disclosed in the Hollister patent, a heavier conductive mesh is required to accomplish effective shielding because of the reduced skin effect at lower frequencies. A heavier conductive mesh is undesirable not only because more light output is blocked, but also because the cost is increased.
While high pressure electrodeless lamps powered by high frequency power in a termination fixture give generally satisfactory results and have extremely long life, these light sources have certain disadvantages. Starting is relatively slow and several seconds may be required to reach full light output. In addition, starting assist devices are required to initiate the discharge as shown in U.S. Pat. No. 3,997,816 issued Dec. 14, 1976 to Haugsjaa et al., U.S. Pat. No. 4,041,352 issued Aug. 9, 1977 to McNeill et al., and U.S. Pat. No. 4,053,814 issued Oct. 11, 1977 to Regan et al.
The following United States patents relate generally to electrodeless light sources utilizing a high pressure discharge in a termination fixture and may be of interest.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,943,401 Haugsjaa et al March 9, 1976 3,943,402 Haugsjaa et al March 9, 1976 3,993,927 Haugsjaa et al Nov. 23, 1976 3,995,195 Haugsjaa et al Nov. 30, 1976 4,001,631 McNeill et al Jan. 4, 1977 4,001,632 Haugsjaa et al Jan. 4, 1977 4,002,944 McNeill et al Jan. 11, 1977 4,063,132 Proud et al Dec. 13, 1977 4,065,701 Haugsjaa et al Dec. 27, 1977 4,070,603 Regan et al Jan. 24, 1978 ______________________________________